1. Field of the Invention
This invention relates to aluminum oxide layers in integrated circuits, fluorescent lamps, flat panel displays, and other electronic and electrooptical devices, and more particularly to nonaqueous organic precursors for making such aluminum oxides and methods for making such precursors and aluminum oxide layers.
2. Statement of the Problem
A fluorescent lamp typically comprises a cylindrical glass tube, or envelope, containing a fill gas of mercury vapor and a phosphor layer covering the inside of the tube wall. Often, a transparent conductive layer is located between the inner surface of the glass tube wall and the phosphor layer. A protective layer of aluminum oxide, often called alumina, is located between the conductive oxide layer and the phosphor layer to inhibit or delay discoloration and other appearance defects in the phosphor layer and the conductive oxide layer. A conventional technique of the art of forming transparent aluminum oxide layers in fluorescent lamps involves: dispersing a solid powder of aluminum oxide in a liquid medium to make a colloidal suspension of the oxide; applying a coating of the suspension onto a surface of the lamp; and drying the coating to form the aluminum oxide layer. Generally, it is difficult to achieve a uniform, continuous thin film by applying a colloidal suspension of powdered particles. Another technique involves dissolving a metal alkyl precursor compound in a solvent and spraying the precursor solution onto a hot surface having a temperature above the crystallization temperature of aluminum oxide, whereby the precursor compound is immediately pyrolyzed. It is generally difficult to form a uniform, continuous aluminum oxide thin film by the conventional pyrolysis method of the prior art because pyrolysis of the sprayed precursor compound on the hot substrate results in a broken, uneven surface on the microscopic level.
Transparent thick films of metal oxide are used as dielectric layers and buffer layers in flat panel displays. Using conventional techniques, it is difficult to achieve thick film layers having uniform thickness. In order to form a thick film layer that sufficiently adheres to the underlying substrate, it is necessary to form a coating from metal oxide powder and then heat the coating above the sintering temperature of the metal oxide. High process temperatures are usually incompatible with other materials commonly used in flat panel displays.
Thin films of aluminum oxide are formed in integrated circuits for use as protective xe2x80x9ccapxe2x80x9d layers, diffusion barrier layers, and dielectric insulators. An aluminum oxide capping layer formed on an underlying metal layer, such as aluminum or gold, protects the metal layer against xe2x80x9cnotchesxe2x80x9d and xe2x80x9cmouse bitesxe2x80x9d, which are caused during etching of the wafer during typical fabrication steps. Aluminum oxide also functions well as a diffusion barrier against high-performance metals, such as copper, gold, and silver, which can be used as metallization layers and local interconnects in integrated circuits. Aluminum oxide may be deposited on integrated circuit substrates to function as insulating layers and to fill insulator gaps and recesses. Very thin films of aluminum oxide function well as insulators and diffusion barriers. Thin films in the range of from 5 nm to 300 nm are typically desired in integrated circuits. Aluminum oxide adheres well to common semiconductor substrates, such as silicon and silicon oxide. Often a layer of aluminum is deposited, which is then oxidized by thermal oxidation. Anodization of aluminum using electrolytic solutions is also used in conventional processes. Aluminum oxide in integrated circuits is also commonly formed using sputtering and evaporation techniques. These conventional processes are complex and do not reliably produce structurally and chemically uniform aluminum oxide films.
For the above reasons, it would be very desirable to have a method of forming aluminum oxide that provides uniform thin and thick films without defects and with good electronic properties, and which lends itself to a commercial manufacturing environment.
The present invention provides a method of forming an aluminum oxide thin or thick film utilizing a liquid precursor containing aluminum. The invention further provides novel nonaqueous organic precursors for forming aluminum oxide films, and methods of making such precursors. The invention may be used for forming aluminum oxide layers in an integrated circuit, a fluorescent lamp, a flat panel display or other electronic or electrooptical device.
One embodiment of the invention is a nonaqueous organic precursor comprising an aluminum organic liquid precursor solution (xe2x80x9cprecursor solutionxe2x80x9d) for forming an aluminum oxide thin film layer. In this embodiment, the liquid precursor is a solution in which one or more aluminum organic precursor compounds are dissolved in one or more organic solvents. A liquid precursor solution of the invention is usually applied to a substrate surface using a liquid source deposition technique. The liquid precursor solution contains one or more aluminum organic precursor compounds that lead to formation of the desired aluminum oxide layer upon reaction and crystallization on the substrate surface. The precursor is applied to the substrate surface and treated, usually by one or more heating techniques. As a result, the organic precursor compound or compounds react to form a solid layer having the desired composition on the substrate surface.
A second embodiment of the invention is a nonaqueous organic precursor containing a suspension of aluminum oxide (xe2x80x9cprecursor suspensionxe2x80x9d) for forming an aluminum oxide thick film layer. In this embodiment, the precursor suspension comprises both a suspension of aluminum oxide powder and an aluminum organic precursor solution. The aluminum oxide powder is suspended in an organic liquid medium. The aluminum organic solution includes one or more aluminum organic precursor compounds dissolved in one or more organic solvents. The organic liquid medium of the precursor suspension is selected to be identical with or soluble in the organic solvent of the aluminum organic solution. The aluminum oxide powder, the aluminum organic precursor solution, and the organic liquid are mixed to form an inventive precursor suspension. The resulting slurry of the precursor suspension provides a chemically stable, long-lived suspension suitable for depositing a coating on a substrate by screen printing or other thick film deposition methods. The aluminum organic precursor compound or compounds dissolved in the organic liquid medium of the precursor suspension are dispersed throughout the coating. The deposited coating is heated at relatively low temperatures, less than the sintering temperature of aluminum oxide. Thermal decomposition and reaction of the aluminum organic precursor compounds that permeate the aluminum oxide powder in the slurry coating result in formation of a homogeneous aluminum oxide solid thick film and interconnected uniformly homogeneous aluminum oxide solid thick film. The presence of organic compounds in the deposited coating that react to form aluminum oxide upon treating promotes necking which facilitates the low temperature formation of a continuous and highly cohesive phase of the solid. Without such necking, a continuous cohesive solid can be formed only by approaching or reaching the melting point of the solid. In addition, utilizing the prior art methods that start with a powder, significant voids remain in the solid even after sintering. The reaction of the organic compounds, on the other hand, results in a lesser number of voids, and, in fact, tends to fill any voids present, resulting in better breakdown strength as well as enhanced electrical and other physical properties. The necking also assists in adhering the aluminum oxide material to the underlying substrate.
A layer of aluminum oxide, in particular a thick film of aluminum oxide, is useful as a buffer layer in flat panel displays. A layer of aluminum oxide has a lower leakage current and a higher breakdown voltage than lead oxide having the same layer thickness. Thus, both thin film layers and thick film layers containing aluminum oxide are useful in flat panel displays, serving as insulators or protective coatings in the display. An inventive precursor suspension is especially useful for forming a thick film containing aluminum oxide because the aluminum organic precursor compounds in solution permeating the coating give rise to a homogeneous material and good adhesive xe2x80x9cneckingxe2x80x9d to the substrate.
In the fabrication of an aluminum oxide layer, a substrate, such as a lamp envelope, may be dipped or rolled in liquid precursor to form a liquid coating of precursor, which is then treated; the liquid precursor solution may also be applied using a liquid spraying method, which is typically used in the fluorescent lamp art. In the fabrication of aluminum oxide thin film layers in integrated circuits, flat panel displays, fluorescent lamps and other electrooptical devices, an inventive liquid precursor solution may be applied by a liquid misted deposition method, in which a very fine mist of liquid particles is formed in a carrier gas and deposited on the substrate surface.
According to the invention, the liquid precursor solution or a precursor suspension is applied to the substrate surface, and then a solid metal oxide is formed in heating steps subsequent to the precursor application step. Electrical properties of an aluminum oxide film can be improved by adjusting the temperature and the duration of the heating steps.
Precursors according to the invention can be manufactured reliably. Their composition can be easily controlled and varied, if necessary. They are chemically stable, so they can be prepared in advance and stored safely for relatively long periods, up to six months. They are relatively nontoxic and nonvolatile, compared to precursors of the prior art. Metal oxide thin film layers formed using liquid precursors of the invention have smooth, continuous and uniform surfaces, especially compared to oxide layers of the prior art. They can be reliably fabricated to have thicknesses in the range of 5 nm to 1000 nm, maintaining important characteristics such as transparency and desired electrical properties.
Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.